21 Advancements in the Proterozoic tectonics of western North America

This session highlights recent developments in understanding of the Proterozoic tectonic record of western Laurentia, particularly its relation to critical mineral resources. Insights derived from all disciplines and approaches are welcome.

Tuesday, 19 May 2026
9:00 AM - 12:00 PM
Alvarado F

Channelized flow or vertical ascent in the Wet Mountains: Do crustal position, fluid activity, and strain partitioning control melt mobility during partial melting of thickening Mesoproterozoic crust?

Paper number: 21-2
Program: Technical Sessions
Time: 9:25 AM – 9:45 AM

Authors

Abstract

Understanding of melt generation, transport, and accumulation in the continental lithosphere is rapidly advancing. Migmatites provide insight into partial melting of continental crust, but extensive migmatite suites are uncommon. Colorado’s Wet Mountains expose a Mesoproterozoic migmatite suite that includes restites, plutons, and dike/sill intrusions.

We present kinematic, microstructural, and stable isotope investigations into Wet Mountains migmatites. We characterize the extent to which migmatites across the range record melt generation, vertical ascent, lateral transport, or accumulation.

Migmatites in the southern Wet Mountains are neosome-dominated and show evidence of melt formation by biotite dehydration. Biotite is variably preserved and most abundant in samples with extensively resorbed garnet. We hypothesize biotite-rich samples retained melt and underwent extensive back-reaction, whereas biotite-poor samples preserve subhedral anatectic garnet and are relatively melt-depleted. Leucosome domains in the south range from centimeters to tens of meters in scale. Ductile flow is broadly distributed, and mylonitic shear is not widely preserved. Vein and dike networks localized plastic shear, which we interpret to support both lateral pure shear and vertical melt ascent.   

In the northern Wet Mountains, oriented mats of biotite define gneissic foliations. Medium to coarse grained, weakly-to-unfoliated veins are variably aligned with or shallowly oblique to foliation in fine-grained biotite, garnet, and sillimanite-bearing gneisses. Gneisses and leucocratic veins are interpreted as residue and melt, respectively. Mylonitic shear commonly overprints foliation-parallel veins. Three distinct vein textures are preserved, with garnet present in only one vein generation. Macroscopic textures indicate pure to subsimple shear. Melt transport is interpreted to be parallel to the gneissic foliation with limited vertical mobility through oblique fractures.

Oxygen isotope ratios of garnet from melt and residue domains suggest anatexis of heterogeneous crust, including sedimentary protoliths (𝛿18O = 8.5 to 11.1‰ [(VSMOW])) and crust previously involved in high temperature hydrothermal systems with access to surface-derived fluids  (𝛿18O = 2.1 to 2.5‰). 𝛿18O values can be achieved through partial melting of a thickened crust and do not require mantle-derived melt in extensional tectonics or arc settings. 

Because melt production, transport, and accumulation depend on crustal position, lithostatic and differential stress, and fluid flow, we propose a tectonic framework in which this melt system developed through strain partitioning and anatexis in thickening crust along the Mesoproterozoic collisional margin of North America.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410713

© Copyright 2026 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.